Routing between ppp0 <-> eth0

Hi,
we want to use the devboard_lx as routing device between an incoming PPP
connection and outgoing ethernet connection.
How can we enable that routing path. At the moment we can make a ping to
the dev_board over PPP but eth0 devices are not reachable.
notebook <--ppp->Dev_board //ping OK
Dev_board <--eth0--> WEB //ping OK
Notebook<--ppp-->Dev_board<-->eth0<-->WEB //ping not ok
At standard Linux we can enable routing by editing
/proc/sys/net/ipv4/ip_forward to 1. On the dev_board that file is not
available.
Regards
Marc